The quest for understanding lymphocyte migration challenged multiple laboratories for several decades now. The development of multi-photon microscopes has shed light on the dynamics of lymphocyte movement in secondary lymphoid organs. In contrast, very little is understood about the dynamics and cellular interactions instructing hematopoietic cell differentiation in bone marrow (BM). B lymphocytes develop from hematopoietic stem and progenitor cells within essential, yet poorly characterized, BM stromal cell niches. A significant body of work has focused on the lymphocyte-intrinsic mechanisms critical for lymphocyte development. Despite their importance, these studies led to lymphoid-centric working models that are insufficient for understanding the complex interactions between hematopoietic precursors and the BM stromal cells that instruct cell lineage decisions. Furthermore, there are currently no conceptual or mechanistic frameworks explaining the development and maintenance of lymphoid-supportive stromal cell niches in BM. In this grant we provide groundbreaking preliminary evidence for a critical role played by the chemokine receptor CXCR4 in BM stromal cells for the development of B-lineage supportive stromal cell niches. In aim 1, we propose to fully characterize the impact of CXCR4-deficiency, exclusively in BM stromal cells, on the maintenance and differentiation of wild-type hematopoietic stem cells. We will also determine how CXCR4 signaling instructs stromal niches to support B cell development in BM. In aim 2, we provide compelling evidence that hematopoietic precursor positioning in lymphoid-supportive stromal niches is a highly regulated process vital for B lymphocyte development. We will characterize the stromal cell niches that are critical for B lymphocyte lineage development. Furthermore, we will define guidance mechanisms attracting lymphoid progenitors to such lymphoid-supportive stromal niches. In late stages of B cell development, immature B lymphocytes undergo stringent central tolerance checkpoints in BM niches that eliminate self-reactive B cells by allowing cells to change their receptor specificity a process named receptor editing) or by promoting cell death (a process known as clonal deletion). In aim 3, we describe a strategy for visualizing B-lymphocyte clonal deletion and receptor editing by intravital 2-photon microscopy. We will use this approach to test the hypothesis that B lymphocyte migration and interactions with stromal cell niches enable B cell central tolerance, using available B cell receptor transgenic and knock-in mouse strains as well as established mouse models of the human autoimmune disease systemic lupus erythematosus. Together, the proposed aims will provide a broad conceptual and mechanistic framework for understanding how guidance cues shape the BM stromal cell compartment and influence cell lineage decisions during hematopoietic cell differentiation.